1. Field of the Invention
The present invention relates to an optical flow sensor which cross correlates digitized signals from a plurality of photodetectors that are separated in a direction parallel to a predetermined direction of gas flow and to a method of determining gas flow velocity in a predetermined direction.
2. Description of the Prior Art
Accurate flow rate measurement represents one of the biggest problems in industrial and environmental monitoring for contaminants in gaseous discharges. The problem of airborne pollution has become so acute that virtually all industries of any size are now monitored for compliance with applicable regulations or laws limiting gaseous discharges. While the determination of the concentration of pollutants, contaminants, noxious gases, and toxic gases in a gas flow has reached a satisfactory level of reliability, monitoring of the rate, and hence the volume of gas flow, has heretofore been rather inaccurate. As a consequence. The determination of the total extent of pollutants emitted from any particular facility has heretofore been uncertain.
Current technologies that estimate flow rate often rely upon a point sensor that intrudes into the path of a flowing gas, for example in the chimney or smoke stack of an industrial processing plant. The estimates for flow rate achieved with such instruments are often inadequate for industrial and environmental applications. One problem is that the intrusion of a sensor head into the flow medium, alters the resulting flow measurements. Furthermore, the flow medium being monitored is often very dirty and corrosive. Direct contact of the sensing head with the flow medium often quickly fouls the sensing head so that maintenance of the sensor is a major problem.
Attempts have been made to devise ultrasonic gaseous flow measuring devices. However, ultrasonic gaseous flow measuring instruments are very expensive and are also rather inaccurate.
Since pollutants and contaminants often provide a distinctive color of conglomerated dust particles to a gaseous discharge, it is sometimes possible to visually monitor chimneys and smokestacks from a distance in order to ascertain a general estimate of rate of gaseous discharge. However, this requires a direct line of sight to the facility involved. Also, some noxious and toxic gases, such as carbon monoxide, are colorless. Moreover, visual observation is not an option at night or in inclement weather. Unfortunately, a monitored industrial facility may limit the volume of its gaseous discharges during the daytime only to increase them at night when the volume of gaseous discharge cannot be observed.
The present invention adopts an entirely new approach to measurement of the velocity of a gaseous flow. The invention utilizes an optical transmitter and an optical receiver located on opposite sides of a gas flow passageway, such as a chimney or smokestack. A beam of light is transmitted across the gas flow passageway. The receiver includes a plurality of photodetectors longitudinally separated from each other in a direction parallel to the predetermined direction of gas flow through the passageway. Particulate matter and eddies in the flowing gas produce scintillations in the beam of light transmitted across the passageway.
These scintillations are detected by all of the photodetectors on the opposite side of the passageway, but detection does not occur at the same time in each photodetector. Rather, since the particle or eddy that creates the scintillation is traveling downstream in the gas flow, the photodetector located furthest upstream will detect the scintillation earlier in time than a photodetector located further downstream. This physical phenomenon is similar to the moving shadow cast by a bird or a piece of debris being carried along on the wind. Temporal cross correlation analysis is then performed on the digitized signals in a digital signal processor utilizing a fast correlation algorithm in which the total number of calculations is proportional to 2N, as contrasted with conventional systems in which the number of correlations is proportional to N2.
One major challenge of calculating cross correlation of two data strings with N time delays is that conventional cross-correlation techniques need a number of calculations proportional to N2. When N is a large number, these calculations are extremely time consuming. To speed up the calculation of the cross-correlation function, a Fast Correlation Algorithm (FCA) according to the present invention has been developed as follows.
1. For two data strings with N (=2n) data points, i.e., A1, A2, . . . AN, and B1, B2, . . . BN, the correlation C1 can be calculated as the ensemble average of the product of Ai and Bi+1, where i=1 . . . Nxe2x88x921. This calculation needs a total of N multiplications.
2. By averaging the two adjacent points of both strings to form two new strings each with length N/2. The correlation C2 can be calculated with N/2 multiplications.
3. Averaging the two adjacent points of the previous two strings once again to form another two strings each with length N/4. The correlation C4 can be calculated with N/4 multiplications.
4. Iteratively doing this process n (=log2N) times. A total of n strings formed each with the data length of N, N/2, N/4 . . . 4, 2, respectively.
From these calculations, the cross correlations C1, C2, C4 . . . CN/2 can be obtained to retrieve the crosswind. The major advantage of the Fast Correlation Algorithm (FCA) is that the total number of calculations is proportional to 2N instead of N2. This represents a reduction by a factor of N/2 in the number of calculations required for cross correlation. This crucial reduction, especially when N is a large number, allows the presently commercially available two-channel digital-signal-processors (DSP) to be utilized since they are fast enough to process the crosswind retrieval algorithm due to the reduced number of calculations that are required.
In one broad aspect, the present invention may be considered to be an optical flow sensor for measuring gas flow velocity in a predetermined direction of gas flow. The optical flow sensor of the invention is comprised of an optical transmitter, an optical receiver, signal-controlled gain amplifier circuits, analog-to-digital converters, and a digital signal processor. The optical transmitter generates a collimated optical beam across the predetermined direction of gas flow. The optical receiver includes a plurality of receiving lenses all located in optical communication with the optical transmitter and in the path of the optical beam. The receiving lenses are separated from each other in a direction parallel to the predetermined direction of gas flow. A separate optical photodetector is provided for each of the receiving lenses. Each photodetector produces an electronic output that varies in response to scintillations occurring in the optical beam.
A separate signal controlled gain amplifier circuit is coupled to each of the optical photodetectors. A separate analog-to-digital converter is coupled to each of the gain amplifier circuits for separately digitizing outputs from each of the gain amplifier circuits.
A digital signal processor is coupled to receive inputs from all of the gain amplifier circuits. The digital signal processor determines the temporal cross variance of the digitized outputs for scintillation events detected separately by all of the photodetectors. Cross correlation is performed using a logarithmic time delay function based upon an original data string with N=2n data points. The fast correlation algorithm averages successive data points in pairs to form a new data string having a length half that of the original data string. The fast correlation algorithm averages each two adjacent points of the new data string to form another data string having a length half that of the data string immediately prior thereto and iteratively averages adjacent data points n=log2N times to provide therefrom a path integrated flow rate in the predetermined direction of gas flow. A digital signal processor can thereby be used to provide a path integrated flow rate in the predetermined direction of gas flow. Since the number of calculations required to determine cross correlation is reduced by a factor of N/2, compared to conventional cross-correlation techniques, a conventional two-channel digital signal processor can be employed in preferred embodiments of the invention.
Preferably, the optical transmitter includes a single laser diode or an LED (Light-Emitting-Diode) while only two photodetectors are employed in the receiver. Preferably also, the optical transmitter includes a collimating lens having a diameter of about one inch focused on the laser diode or LED, and each of the receiving lenses is about two inches in diameter and is focused on its photodetector. In the preferred embodiment of the invention the collimating lens produces a collimated optical beam.
In another broad aspect, the invention may be considered to be an optical flow sensor for measuring velocity of gas flow in a predetermined gas flow direction. The optical flow sensor is comprised of a transmitter, a receiver, signal-controlled gain amplifiers, analog-to-digital converters, and a digital signal processor. The transmitter includes an optical source and beam-forming optics for producing a collimated beam across the gas flow. The receiver is located in a line-of-sight path with the optical beam. The receiver includes a plural number of focusing receiving lenses. The receiving lenses are spaced apart from each other in a direction parallel to the predetermined gas flow direction. A separate photodetector is provided for each of the receiving lenses. Each photodetector produces an electronic output that varies with scintillation events occurring in the line-of-sight path. Separate signal controlled gain amplifiers are provided for each of the photodetectors. Separate analog-to-digital converters are coupled to each of the photodetectors for converting their outputs from analog to digital signals.
The digital signal processor cross correlates the digitized signals from the photodetectors as a function of time delay between the detection of scintillation events based upon a number of time delays=N using a fast correlation algorithm which is a fast Fourier transform in which the total number of calculations of cross correlation data points is proportional to 2N. This allows a conventional, commercially available digital signal processor to be used in preferred embodiments of the invention to produce a signal indicative of velocity of gas flow in the predetermined gas flow direction.
In still another broad aspect, the invention may be considered to be a method of measuring velocity of gas flow in a predetermined linear direction. The steps of the invention include: transmitting a collimated optical beam across a volume of gas along an optical line-of-sight path intersecting the predetermined linear direction of gas flow and crossing the gas flow; receiving the transmitted optical beam with a plurality of receiving scintillation detectors spatially separated from each other in a direction parallel to the predetermined linear direction; generating a separate output signal from each of the receiving scintillation detectors in response to scintillations occurring in the gas flow; separately amplifying and digitizing each of the output signals; determining the temporal cross correlation of the separate output signals by using a logarithmic time delay function based upon an original data string with N=2n data points by averaging successive data point in pairs to form a new data string having a length half that of the original data string and averaging each two adjacent points of the new data string to form another data string having a length half that of the new data string, and iteratively averaging adjacent data points n=log2N times; and providing a velocity output signal indicative of velocity of gas flow in the predetermined linear direction based upon distance of separation of the receiving scintillation detectors from each other and upon the temporal cross correlation of the separate output signals.
The method of the invention preferably also includes the step of calculating a time averaged mean output magnitude of signals from each of the photodetectors, repetitiously comparing the time averaged mean output magnitude with signal magnitude from each of the photodetectors, generating a series of single bit codes for the signals of each photodetector depending upon signal magnitude relative to the time averaged mean output magnitude, and utilizing the series of single bit codes for each of the photodetectors in the step of determining temporal cross correlation of the separate output signals.
In operation, turbulence eddies pass through the beam and modulate it. This optical scintillation is received by a pair of adjacent detectors located in the receiver unit housing. The sensor computer calculates the temporal cross correlation of the signals received by the pair of receiving optics. The path integrated flow rate is then derived. Determination of the temporal cross correlation can be implemented by a digital signal processor (DSP). To efficiently use a DSP in a binary domain, it is best to use a one-bit correlation technique to obtain the time-delayed covariance function of the detected signals from the two photodetectors. A one-bit correlation technique is adequate for data processing to retrieve the flow rate from the two detected optical scintillation signals. For DSP programing, one-bit operation is much faster than multi-bit operation, and it also needs less memory.
By taking the difference of positive and negative time delays of the covariance function of the scintillation signals received from the two detectors, i.e., Cd(r,t)=CXr,t)xe2x88x92CX(r,xe2x88x92t), all the noises common to both detectors will be removed. The width of the difference covariance function Cd(r,t) is inversely proportional to the path-averaged flow rate. However, the width of the difference covariance function Cd[r,log(t)] is independent of the path-averaged flow rate if Cd[r,log(t)] is plotted on a logarithmic scale of time delay. The amplitude of Cd[r,log(t)] will depend on the signal-to-noise ratio only and it is not a function of flow rate velocity. Therefore the amplitude or the area under the curve Cd[r,log(t)] can be used as a data quality factor to indicate the level of signal-to-noise (S/N). Larger amplitude or area indicates better S/N. This S/N quantity can be used as a data quality check.
The sensor will provide real-time continuous measurements of the flow rate in field operation. The instrument is insensitive to environmental acoustic and electromagnetic noises. Unlike most of the traditional flow sensors, one major advantage of this technology is that the measured flow rate is independent of both temperature and pressure. Therefore, the measured velocity is a true measurement regardless of the ambient pressure and temperature of the flow medium.
Preferably the series of single bit codes from each of the photodetectors is analyzed to determine peaks in the output signals from each of the photodetectors. This information is used to ascertain the difference in time of detection of identifiable scintillation events by each of the photodetectors.
In the preferred practice of the invention, the signal-to-noise ratio is calculated for the output signals of the photodetectors as a data quality check. Preferably also the output signals of the photodetectors are subjected to threshold screening to improve reliability of the velocity output signals.
The present invention provides an innovative optical system that measures real-time flow rate of a flowing fluid without employing sensors that intrude into the flow media. The present invention also provides a relatively inexpensive and very reliable flow rate measuring system that operates unattended day and night.
It should also be noted that time delay range of covariance function is critical for crosswind retrieval. Measurement of slower speeds of fluid flow requires longer time delays than measurement of faster speeds.
The invention may be described with greater clarity and particularity by reference to the accompanying drawings.